(1) Field of the Invention
The instant invention is directed to an improved optical trains alignment of optical fiber communication channels in a rotary joint assembly, and more particularly, to such an assembly which enables the propagation of signals on multiple, single-mode or multi-mode, optical fiber channels across a continuous rotary interface with low loss, low crosstalk between channels, and low reflection of light back into the input fibers.
(2) Description of the Prior Art
The apparatus of the present invention especially satisfies a requirement arising in connection with multiple channel optical fiber rotary joints. Multiple channel rotary joints for multi-mode fibers have been developed but exhibit high optical loss and variation of loss with rotation. Additionally, the extreme tolerances associated with single-mode fibers preclude the easy adaptation of rotary joints to handle single-mode fiber. The tight tolerances required by single-mode fibers generally result in excessive loss or excessive variation of loss with rotation. Rotary joints for single-mode fiber have been developed, but are limited to a single on-axis fiber. There are devices which actively align the output fibers to track the input fibers, but these are complicated, require electrical power, and do not allow operation in both directions.
Another type of rotary joint, which is a predecessor to the present invention, is exemplified by U.S. Pat. 4,109,998 to Iverson. The Iverson rotary joint utilizes a derotation optical element such as a dove prism to derotate the images of an input set of optical transmitters located on the rotor, so that they may be focused onto stationary photo detectors located on the stator. Derotation is accomplished by gearing the rotor and the prism in such a way that the prism rotates half as fast as the rotor. The Iverson optical rotary joint does not utilize optical fiber, but rather light emitting diodes (LEDS) or lasers and laser detectors. As a result, it does not require the high alignment accuracy required for optical fibers, because the detectors may be quite large. The device is not bidirectional. Because of the relatively large size of the detectors, they will be limited in bandwidth to well below the bandwidth capability of the optical fiber. Several design features preclude its adaptation to optical fiber. The lenses which focus light from input to output are large lenses located on the prism rotor on either side of the prism. While acceptable for the relatively crude alignment of LEDs and the type of detectors disclosed in the Iverson patent, this would be a precluding factor for purposes of an optical fiber application by reason of the latter's requirement to maintain ultra-tight lateral and angular tolerances between rotor, prism rotor and stator. Additionally, some channels are necessarily off the lens axis, leading to unacceptable aberrations in coupling a plurality of optical fibers. In general, because it was not required, the Iverson patent contains no teaching regarding associated mechanical means of meeting and maintaining the critical alignment tolerances imposed by optical fiber.
Typically in the heretofore known rotary joints for multi-mode fibers, ferrule-encased-optical-fiber-terminations were simply mechanically inserted into lens holders for cylindrically-bodied-miniature-collimation-lenses with the encased-fiber-terminations simply concentrically aligned with the respective one of the collimation lenses.